1. Field of the Invention
The present invention relates to timing circuits, and more particularly to precision swept delay circuits. A particular application is to radar timing circuits, including precision swept delay circuits for equivalent time ranging systems. It can be used to generate a swept-delay clock for sampling-type radar, laser and TDR systems, as well as radio and ultrasonic systems.
2. Description of Related Art
High-resolution pulse-echo systems such as wideband pulsed radar, pulsed laser rangefinders, and time domain reflectometers (TDR) generally sweep a timing circuit across a range of delays. The timing circuit controls a receiver sampling gate such that when an bill echo signal coincides with the temporal location of the sampling gate, a sampled echo signal is obtained. The echo range is then determined from the timing circuit, so highly accurate swept timing is needed to obtain accurate range measurements.
Prior art approaches to swept timing include analog methods and systems: (1) an analog voltage ramp that drives a comparator, with the comparator reference voltage controlling the delay, or (2) a delay locked loop (DLL), wherein the delay between a transmit and receive g clock is measured and controlled with a phase comparator and control loop. Both approaches are subject to component and temperature variations, and are generally limited to an accuracy of 0.01 to 1 percent of full scale. Examples of DLL architectures are disclosed in U.S. Pat. No. 5,563,605, xe2x80x9cPrecision Digital Pulse Phase Generatorxe2x80x9d by McEwan, and in copending application, xe2x80x9cPhase-Comparator-Less Delay Locked Loopxe2x80x9d,filed May 26, 1998, Ser. No. 09/084,541, by McEwan, now U.S. Pat. No. 6,055,287.
A potentially more accurate approach uses two oscillators with frequencies FT and FR that are offset by a small amount FTxe2x88x92FR=xcex94. In a radar application, a first oscillator of frequency FT triggers transmit RF pulses, and a second oscillator of frequency FR triggers a short sampling gate for the echo RF pulses. Due to the small frequency difference xcex94, the timing of the sampling gate smoothly and linearly slips in phase (i.e., time) relative to the transmit clock such that one full cycle is slipped every 1/xcex94 seconds. The two frequencies are directly measured and used to control xcex94.
The slow phase slip creates a time expansion effect of FT/xcex94 (xcx9c100,000 typically). Thanks to the expansion effect, events on a picosecond scale are converted to an easily measurable microsecond scale. In contrast, a real time counter would need a teraHertz clock to measure with picosecond resolution, well beyond present technology.
This two-oscillator technique was used in the 1960""s in precision time-interval counters with sub-nanosecond resolution, and appeared in a short-range radar in U.S. Pat. No. 4,132,991, xe2x80x9cMethod and Apparatus Utilizing Time-Expanded Pulse Sequences for Distance Measurement in a Radar,xe2x80x9d issued in 1979 to Wocher et al.
The accuracy of the two-oscillator technique is limited by the accuracy of the clocks, which can be extremely accurate, and by the smoothness, or linearity in phase vs. time, of the phase slip between them. Nothing appears in the prior art to support the linearity of the phase slipxe2x80x94it is not easy to measure, and it is also easy to assume it is somehow perfect. Unfortunately, there are many influences that can affect the smoothness of the phase slip that are addressed by the present invention. These include digital cross-talk that can produce 100 ps of error or more, and offset frequency control circuit aberrations than can introduce even more substantial phase slip nonlinearities.
The present invention is a precise clock system for pulsed radio, radar, laser, ultrasonic, and TDR ranging systems (and other timing applications which need an offset frequency) requiring high stability and accuracy, and a transmitter-receiver system incorporating the clock system. The clock system generates a first clock signal to drive a transmitter and a second clock signal to drive a sampling-type receiver. The present invention is a two oscillator timing system having a first oscillator to provide the first clock signal and a second oscillator to provide the second clock signal. The frequencies (FT, FR) of the two clocks differ slightly (by xcex94) such that a smooth phase slip occurs between them. Thus, a replica of the echo (travelling at the speed of light for electromagnetic systems) is produced by the sampler on a slow time scale (1/xcex94xcx9c40 milliseconds), known as equivalent time, which directly allows high resolution (e.g., picosecond) measurements on an expanded scale. In contrast to the prior art, the frequency difference xcex94 between the two oscillators is not directly measured; instead, an effect arising from xcex94xe2x80x94the receive pulse ratexe2x80x94is measured and controlled.
Key advantages to this arrangement include (1) the first oscillator can be totally isolated from the rest of the system (except its connection to a transmitter), so error-producing crosstalk can be eliminated, (2) the first oscillator can be remotely located, such as in a radio system, (3) a simplified implementation can be realized, since a mixer and frequency divider chain is not required, and the overall embodiment is compact and of low cost.
The present invention uses a sampling-type frequency locked loop (FLL) between the receiver and the second clock to accurately control the slip rate xcex94, and an optional phase lock port is provided to phase lock xcex94 to an external reference frequency xcex94REF. Additionally, the FLL employs a wrong-sideband detector so the FLL can reliably lock to small values of xe2x88x92xcex94 without a false lock at +xcex94, i.e. the FLL will ensure that the second oscillator frequency is slightly lower than FT (i.e., FTxe2x88x92xcex94) rather than slightly higher (i.e., FT+xcex94).
The present invention differs significantly from prior art timing systems based on offset oscillators in that: (1) the FLL locks to the repetition rate of detected receive pulses, (2) a sample-hold type FLL is used to eliminate phase slip nonlinearities, and (3) there is no direct connection between the transmit clock and the receive clockxe2x80x94offset frequency control is routed through the transmit-receive apparatus.
A primary object of the present invention is to provide a high accuracy swept timing circuit for time-of-flight ranging systems.
Yet another object of the present invention is to provide a simple xe2x80x9cplug-and-playxe2x80x9d timing system for highly accurate, low-cost ranging systems.
A further object of the present invention is to eliminate errors due to crosstalk and control loop aberrations.
Applications include low cost radars for security alarms, home automation and lighting control, industrial and robotic controls, automatic toilet and faucet control, automatic door openers, fluid level sensing radars, imaging radars, vehicle backup and collision warning radars, and universal object/obstacle detection and ranging. One specific embodiment utilizing the present invention is a time domain reflectometer (TDR) where a pulse is propagated along a conductor or guidewire to reflect from a material for use in a variety of applications, such as an xe2x80x9celectronic dipstickxe2x80x9d for fluid level sensing.